The present invention relates to a printed wiring board having an electromagnetic wave shielding layer.
Recently, a printed wiring board having a shielding layer has been developed for preventing normal oPeration of a circuit from being interferred with due to electrical noise from external instruments.
An embodiment of such a printed wiring board having an electromagnetic wave shielding layer is concretly shown in FIG. 2. The printed wiring board comprises an electrical insulating sheet 1, a printed wiring circuit 2 provided on one major surface of the sheet, an electrcal layer 3 provided on the printed wiring circuit, an electromagnetic wave shielding layer 4 provided thereon and an over-coating layer 5 provided on the whole surface thereof for protecting the shielding layer.
A contact portion 6 is provided between the electromagnetic wave shielding layer 4 and grounded wiring circuits 2a for electrically contacting therebetween.
This contact portion 6 between the shielding layer 4 and grounded wiring circuit 2a may be provided for any position and at least two positions in the whole printed wiring circuit 2.
In this case, the electromagnetic wave shielding layer 4 of the printed wiring board is formed by using conductive ink which is formed by mixing conductive metal powder with resin ink by applying this conductive ink onto the electrical insulating layer with a silk screen printing method, and by drying and hardening the conductive ink.
On considering characteristics of the conductive ink, a great amount of metal powders must be mixed in the resin in order to increase conductivity. In this case, the resin amount in the conductive ink becomes small, resulting in a decrease of adhesive force to the contact portion.
Particularly, when electrical or electronic components, such as a capacitor are mounted on the printed wiring board, the fixing of the electronic components is performed by the soldering under the condition at about 260.degree. C. for 3.about.10 sec.
In this case, the mounting of such electronic components has an affects on the adhesive force of the electromagnetic wave shielding layer 4 with a thermal shock caused by the soldering. Moreover, the difference in thermal expansion coefficient between the printed wiring circuit 2 of copper foil and the over-coating layer 4 in the printed wiring board becomes remarkable at the contact portion 6 of the electromagnetic wave shielding layer 4 under high temperature conditions, thereby causing a pealing-off of the contact portion.